The present invention relates to a seismic prospecting method and device utilizing jogs generated by a drill bit working at the bottom of a well.
According to a commonly used technique, well drilling is achieved by driving, through a motorized rotary table arranged at the surface, a drill column made up of a string of interconnected pipes and provided with a drill bit at the lower end thereof.
Drill bits of this type generate powerful seismic waves and it has been envisaged to use these waves for carrying out measuring operations in the formations crossed by the drill bit during the drilling process and notably seismic prospecting operations (referred to as MWD operations), in order to decrease oil prospecting costs since the wells are not immobilized thereby. However, this type of operations is difficult to implement for many reasons. The amplitude of the jogs produced by a drill bit depends to a large extent on the hardness of the formations crossed. The signals are transmitted permanently by the bit with no possibility of fixing a reference instant. Besides, the frequency spectrum emitted depends on many factors: the type of drill bit used, the rotary speed thereof, the various mechanical elements constituting the drill string, the nature of the formations crossed, etc.
The form and the emission spectrum of the source are generally not well-known because of the distance between the point of emission and the surface station. The jogs are most often picked up by accelerometers arranged in contact with the tubular drill string in the vicinity of the surface after propagating mechanically all along the latter. Moreover, the signals transmitted are strongly distorted by complex resonance and absorption phenomena all along the string and they depend on the structure of the surface installation. As the transfer function of the assembly is not well-known, rigorous references for processing the seismic signals are therefore not available.
Another well-known method consists in fixing a sensor to the base of the drill pipe so as to generate a signal representative of the jogs emitted and for transmitting them to the surface through modulated waves, such as acoustic waves propagating in the fluid contained in the well. Such a method is used when the rate of the data to be transmitted is relatively low, but it is not suitable within the scope of prospecting operations where the volume of acquired signals is high, unless very powerful bottomhole storage means are available.
Interpretation difficulties also appear because the jogs are not emitted only at the level of the drill bit. It has been observed that many locations along the string act as secondary jog sources. This is also the case of the surface drilling platform, which transmits to the formations, through the legs thereof, mechanical energy which propagates along the string and acts therefore also as a secondary jog source.
Implementation examples in this field are described in U.S. Pat. Nos. 4,965,774; 4,926,391; 4,718,048; 4,675,852, etc.